The prognosis for premature infants has steadily improved as our knowledge of their metabolic requirements increases. The nutritional needs of these infants are distinct from those of their full-term counterparts. Likewise, endocrine regulation is unique, as evidenced by "immature" feedback regulation of TSH by thyroid hormone (TH) in preterm neonates. A large body of data indicate the advantages of premature maternal milk (PMM) for the preterm infant, and the rate of breast feeding among these mothers has risen dramatically. Analysis of the composition of PMM and how it affects the neonate is thus mandatory to successful management of these infants. Our studies and others have confirmed the presence of TH in term breast milk. Elevated TH levels in breast-fed full- and pre-term infants have been reported. In order to define the thyroidal components of PMM and further, evaluate their influence upon the immature thyroid economy of the preterm infant, we propose to: (1) Isolate, identify, and quantify: (a) T3 and T4 in PMM at 0, 2, and 4 wks lactation using high performance liquid chromatography (HPLC) and radioimmunoassay (RIA); and (b) thyroid hormone analogues and thyromimetic amino acids in PMM by HPLC coupled with post-column derivatization, in light of their potential bioactivity. (2) Quantify levels of TSH in PMM by ConA-Sepharose chromatography and TSH-RIA; and characterize thyroxine-binding capacity by (i) RIA, (ii) isoelectric focusing and electrophoresis for physical characterization; and (iii) equilibrium dialysis to determine binding properties. (3) Determine how thyroid status of PMM-fed infants differs from that of formula-fed preterm neonates, by assessing serum T4, T3, rT3, TSH, and TBG concentrations in light of maternal serum and milk levels. (4) Investigate further absorption by the immature gut of peroral TSH, examining: (a) effect of gut maturation upon T3 and T4 responses to thyrotropin stimulation in 7-15 day old rat pups; (b) dose-responsiveness of the suckling rat thyroid; and (c) gut-induced alterations of bio- and/or immuno-reactivity of circulating thyrotropin resulting from peroral ingestion of TSH, by Sephadex chromatography, rTSH-RIA, and bioassay of adenylate cyclase activity. The proposed studies encompass the fields of neonatology, endocrinology, biochemistry, gastroenterology and physiology. The data generated by these investigations will lead to a greater understanding of the extent to which the changing hormonal requirements of the preterm infant may be satisfied by the dynamic changes in breast milk composition during the course of lactation.